The invention relates to a process for comminuting silicon and also the use of comminuted silicon as active material in lithium ion batteries.
Such a process makes it possible to produce microsize and nanosize silicon particles which are suitable as active material in anodes of lithium ion batteries.
Rechargeable lithium ion batteries are at present the practically usable electrochemical energy stores having the highest energy densities of up to 250 Wh/kg. They are utilized mostly in the field of portable electronics, for tools and for transportation means, for example bicycles or automobiles. However, particularly for use in automobiles, it is necessary to achieve a significant further increase in the energy density of the batteries in order to achieve longer ranges of the vehicles.
As negative electrode material (“anode”) in lithium ion batteries, use is made mostly of graphitic carbon. Graphitic carbon has stable cyclic properties and quite high handling safety compared to lithium metal which is used in primary lithium cells. An important argument for the use of graphitic carbon in negative electrode materials is the small volume change of the host material associated with intercalation and deintercalation of lithium, i.e. the electrode remains approximately stable. Thus, a volume increase of only about 10% is measured for intercalation of lithium in graphitic carbon for the limiting stoichiometry of LiC6. However, a disadvantage is its relatively low electrochemical capacity of theoretically 372 mAh/g of graphite, which corresponds to only about one tenth of the electrochemical capacity which is theoretically achievable when using lithium metal.
Silicon and lithium form binary electrochemically active compounds which have a very high lithium content. The theoretical maximum lithium content is found in Li4.4Si, which corresponds to a very high theoretical specific capacity of 4199 mAh/g of silicon. The disadvantage of silicon as anode material is that the intercalation and deintercalation of lithium is associated with a very large volume change, which is up to 300%. This volume change leads to severe mechanical stressing of the crystallites and thus to rupturing of the particles with loss of electrical contact.
EP 1730800 B1 discloses, for example, an electrode material for lithium ion batteries, characterized in that the electrode material comprises 5-85% by weight of nanosize silicon particles which have a BET surface area of from 5 to 700 m2/g and an average primary particle diameter of from 5 to 200 nm, 0-10% by weight of conductive carbon black, 5-80% by weight of graphite having an average particle diameter of from 1 μm to 100 μm and 5-25% by weight of a binder, where the proportions of the components add up to a maximum of 100% by weight.
WO 13040705 A1 discloses a process for producing particulate material for use in anodes, which comprises dry milling of particles of an element of the carbon-silicon group to give microsize particles, wet milling of the microsize particles dispersed in a solvent to give nanosize particles (10-100 nm). It is provided for the nanoparticles to be mixed with a carbon precursor and the mixture to be pyrolized in order to coat the nanoparticles at least partly with conductive carbon.
A process which has been known for a long time for producing Si nanoparticles is wet milling of a suspension of Si particles in organic solvents by means of a stirred ball mill filled with ceramic or steel milling media (T. P. Herbell, T. K. Glasgow and N. W. Orth, “Demonstration of a silicon nitride attrition mill for production of fine pure Si and Si3N4 powders”; Am. Ceram. Soc. Bull., 1984, 63, 9, p. 1176). It is stated in this publication that reactions of the material being milled with the suspending liquid can take place during milling. Moreover, the milled material is, depending on its initial and final size, typically contaminated with a few percent by weight of foreign atoms by contact with the mill and in particular wear of the milling media. Silicon is a hard and brittle material for which ceramic milling media such as milling beads composed of yttrium-stabilized zirconium oxide are frequently used. After milling, zirconium can easily be detected in the silicon dispersion by ICP-OES.
Metallic impurities in the electrode may have an adverse effect on the electrochemistry, which can result in the capacity of the battery decreasing significantly over a number of charging and discharging cycles. This effect is referred to in the literature as fading and irreversible loss of capacity.
U.S. Pat. No. 7,883,995 B2 claims a process for producing stable functionalized nanoparticles smaller than 100 nm, with the particles being functionalized during milling in a reactive medium in a ball mill. Alkenes, in particular, are used for functionalizing the particle surface because the double bonds can react particularly easily with open bonds on the fracture surfaces of the particles.
A further known industrial process for producing particles having low contamination by wear from milling media is autogenous comminution in a stirred mill.
EP 0700724 A1 discloses a process for the continuous autogenous milling of a flowable treatment material containing insoluble particles of differing diameters, wherein the treatment material is set into rotation concentrically to an axis in a milling space and wherein insoluble particles of greater diameter are overproportionately concentrated at the wall of the milling space compared to particles of smaller diameter.
Use is made here of stirred ball mills which in each case have a cylindrical vessel in which a stirrer which can be driven at high speed is arranged concentrically. The milling vessel is at least substantially filled with milling media. The treatment material is fed in flowable form, for example as a slowing water, into the vessel at one end and leaves the vessel at the other end. The mixture of treatment material and milling media is set into intensive motion by the stirrer so that intensive milling takes place. The key aspect of the invention is that the particles having a greater diameter are concentrated in the milling space and there are firstly used simultaneously as milling media for milling the particles having a smaller diameter and secondly are themselves abraded on the outside. Thus, no independent milling media of a different type are used. The treatment material is classified outside the mill before milling and sorted into particles having a greater diameter, which are used for milling, and particles having a smaller diameter, which are to be milled. The particles having a greater diameter are introduced into the milling space of the stirred ball mill and remain here. Subsequently, only the preclassified treatment material comprising particles having a smaller diameter is passed through the milling space in order to be milled. Disadvantages of this process are that the milling performance decreases sharply for small particles below one μm and special mills with additional centrifugal separation systems are necessary in order to separate the fine milled material from the coarser milling media. The engineering outlay for autogenous comminution increases significantly with decreasing particle size.
In autogenous milling, milling media composed of the same material as the material being milled are used.
These are generally relatively coarse particles or fragments from a previous milling stage.
However, many materials are not suitable at all as milling media because of their mechanical properties.
U.S. Pat. No. 7,789,331 B2 discloses a process for milling silicon powder by means of a jet mill. The material to be milled is induced to undergo collisions among the particles and with the walls of the mill by gas turbulence, as a result of which the material to be milled is pulverized. It is stated that a particle distribution of from 0.2 to 20 μm was achieved. Polycrystalline silicon granules are also proposed as material to be milled.
However, our own experiments have shown that this particle distribution should be interpreted as meaning that the median of the distribution is about 10 μm (with d10=about 5 μm and d90=about 17 μm). Smaller particles having a size of less than 1 μm are usually at present in an amount of not more than a few percent in the particle distribution. This is related to the fact that during dry milling particles having a size of less than 1 μm agglomerate strongly because of increasing adhesion. For this reason, in dry milling of very small particles, the milling energy is required mainly for breaking up the agglomerates and the actually intended milling of the primary particles themselves essentially no longer takes place. The production of nanosize particles by dry milling of silicon is therefore very difficult to bring about and uneconomical.